Add a Makefile recipe to create multi-lingual dev builds

Dev builds need packaged locales like normal builds, or we have Fluent troubles.

It's quite easy to do it:

export MOZ_CHROME_MULTILOCALE="it es-ES de" # Whatever locales you want to support
./mach package-multi-locale --locales en-US $MOZ_CHROME_MULTILOCALE # No quotes on purpose
AB_CD=multi ./mach build stage-package

However, it's even easier not remembering the commands 😄.

So, we could integrate them in our Makefile, and when we pass a language variable (or when we use a specific target) we could create a localized build.